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This paper proposes the use of ac polarization for resonant electrostatic microelectromechanical systems that eliminates the frequency drift caused by dielectric charging and charge screening. It is mathematically and experimentally shown that an ac-polarized resonator can sustain stable oscillations when used in a positive feedback oscillator circuit. We also demonstrate an oscillator topology that generates a drift-free reference frequency tone with this technique in spite of using a resonator that exhibits large frequency drifts under dc polarization. Long-term data are presented for these drift-susceptible devices, showing a significant improvement in frequency stability.